The present invention relates to an improved method for driving loads with hydraulic actuators.
Hydraulic actuators are used to convert energy such as is found in a pressurized fluid into mechanical motion that can displace a load. The mechanical motion includes a linear push or a pull that produces a change in elevation in a load. The mechanical motion also includes rotary and oscillatory motion.
One type of hydraulic actuator commonly used to provide a fixed length of linear motion is a linear cylindrical actuator. The linear cylindrical actuator includes a closed cylinder and a piston tightly fitted within the cylinder. The piston is attached to a rod extending from one end of the closed cylinder. The rod of the actuator engages and drives the load. The piston is fitted within the cylinder so that the piston can be pushed toward the load or away from the load in the cylinder in response to the rod engaging or disengaging the load.
The piston divides the closed cylinder into two sections, a section including the rod, a "rod section," and a section without the rod, a "rod-free" section. As the piston is moved toward the load, the rod section of the closed cylinder decreases in volume and the rod-free section increases in volume.
The piston is also pushed toward the load or away from the load by pressurized hydraulic fluid in at least one of the rod or rod-free sections of the closed cylinder. At least one of the rod and rod-free sections has a portal for a feed or exhaust of the pressurized hydraulic fluid. The pressurized hydraulic fluid fills at least one of the two cylinder sections through the portal. The hydraulic fluid is typically pressurized by a pump.
The pump is placed in communication with one or more cylindrical actuators by a piping network through which pressurized hydraulic fluid is transferred. One type of piping network is a parallel piping network. In the parallel piping network, a first and second actuating cylinder separately and concurrently receive fluid pressurized by the pump. For a situation where both of the actuators are engaging the same load, the actuators separately receive pressurized fluid through the portal in the rod-free section of each actuator to push the piston and rod of each actuator toward the load. The actuators separately and concurrently exhaust fluid from the rod-free section of each actuator into a tank to move the piston and rod of each actuator away from the load.
One disadvantage of the parallel, concurrent transmittal of hydraulic fluid is that when the pistons are moved toward the load by pressurized hydraulic fluid to cause the rods to approach but not to engage the load, the pressurized hydraulic fluid has an excess energy that is undesirably dissipated as heat. The excess energy arises because the hydraulic fluid is fully pressurized by the pump but the actuators are not engaging and driving a load. A generation of heat by the excess energy is undesirable because heat elevates the temperature of the hydraulic fluid beyond acceptable limits.
Another type of piping network is a series piping network. In a series piping network, pressurized hydraulic fluid is transferred from the pump to one of the sections of the first cylindrical actuator and then to one of the sections of the second cylindrical actuator. A series piping network reduces the volume of hydraulic fluid required to move pistons and rods of the actuators as compared to the parallel network. This is because the separate inlet and exhaust piping required for each actuator in the parallel network is not required in the series network. However, the series network requires that pump pressure be increased over what is required by a parallel network to move a given load. This is because the pump must pressurize a single long network of at least two actuators rather than the shorter separate parallel networks of each actuator.